The present invention relates in general to optical fiber cables. More particularly, the invention relates to an improved optical fiber cable in which the cable core is comprised of variable fiber count optical fiber units for maximizing the packing density of the cable core, and a method of maximizing the packing density of the cable core.
Optical fiber technology, to include the usage of optical fiber cables, has increasingly grown in demand and usage. Optical fiber cables offer the advantage of passing large amounts of data quickly, easily, and with a degree of reliability unmatched by conventional wire data transmission technology. Accordingly as the usage of optical fiber cables increases, it is becoming increasingly difficult to utilize the existing infrastructure to accommodate the demand for increased optical fiber cable capacity. For example, there is only a limited amount of underground duct space currently in existence, and it would be extremely expensive to begin adding additional underground duct space, or duct space in existing buildings, to accommodate the demand for ever greater numbers of fiber optic lines.
For example, limited underground duct space, cable joining costs, and cable manufacturing costs place an economic premium on fiber optic cables with high fiber counts, as well as high fiber packing densities for each measure of cable sheath diameter. For many years, it has been common practice to use a stack of fiber optic ribbons to achieve high fiber counts and packing densities. An example of such an approach to optical fiber cable core construction is illustrated in U.S. Pat. No. 5,878,180 to Nothofer, et al. In the patent to Nothofer et al., an optical fiber cable core is provided with a plurality of superimposed and adjacent stacks of optical fiber ribbons, the stacks of fiber ribbons being arranged over and/or adjacent to each other, and in parallel. As a result, the optical fiber cable core of Nothofer, et al. provides a stack of optical fiber ribbons, in which each of the ribbons has an identical number of optical fiber cables provided as a part thereof.
Other approaches which attempt to maximize the packing density of optical fibers within optical fiber cable cores are disclosed in U.S. Pat. No. 4,906,067 to Mayr, et al., disclosing an optical fiber cable comprised of a plurality of bundled elements, each bundled element having an identical number of optical fibers therein, and in U.S. Pat. No. 5,857,051 to Travieso, et al., which discloses a high density riser and plenum breakout cable for indoor and outdoor cable applications, in which the cable is comprised of optical fiber ribbon structures, the ribbon structures being broken down into four substructures which are stacked within the cable core, and where selected substructures can be broken out of the cable and used where desired.
Ribbon structures of the type utilized in these existing approaches to maximizing the number of optical fibers in an optical fiber cable are disclosed in U.S. Pat. No. 4,900,126 to Jackson, et al., disclosing a bonded array of optical fibers formed as a ribbon; and U.S. Pat. No. 5,905,835 to Bourghelle, et al., disclosing a multi optical fiber ribbon in which two identically sized and shaped ribbons are bounded together for forming one larger ribbon of optical fibers.
One example of a known type of optical fiber cable core configuration which utilizes a stack of optical fiber ribbons, similar to the approach of the patent to Nothofer et al., is illustrated in FIG. 1, which shows in cross-section an optical fiber cable 5 having an elongate continuous core tube 6. Situated within the core tube is a stack 7 of optical fiber ribbons 9, each one of the optical fiber ribbons having an identical number of optical fibers therein. As seen, such a configuration attempts to maximize the packing density of the optical fiber ribbons within the cable core, yet a great amount of free space remains within the cable core. The stack 7 of optical fiber ribbons 9 shown in FIG. 1 is an illustrative configuration only, it being understood that varying square or rectangular stacks of optical fiber ribbons may be present within the core tube, which serves to illustrates the problem, which is that these square or rectangular ribbon stack configurations do not fully utilize the space available within the core tube. It is also understood, although not illustrated in FIG. 1, that the optical fiber cable will be provided with the requisite jacketing, strength members, rip cords and/or water blocking tape which comprise a part of optical fiber cables, as known.
As is known, the maximum number of optical fibers that can be placed within a cable core tube, without incurring excess added loss, is constrained by the dimensions of the stack of optical fibers relative to the inner diameter of the core tube. Specifically, the ratio of the diagonal of the ribbon stack to the core tube inside diameter cannot exceed a predetermined value without inducing excessive cabling and low temperature added optical losses. In the alternative, for a given core tube inner diameter, a minimum clearance between the core tube wall and any optical fiber in the stack may be specified. Although cable cores, such as those disclosed above, comprised of a rectangular stack of optical fiber ribbons offer an attractive solution to the need for maximizing optical fibers within a cable, a rectangular stack of optical fiber ribbons, with each ribbon having a constant fiber count, such as those disclosed in the ""180 patent to Nothofer, et al., the ""051 patent to Travieso, et al., and the ""067 patent to Mayr, et al., do not fully utilize the space available within the cable core for fiber containment, and thus there exists a need for an improved optical fiber cable, more specifically an improved optical fiber cable core, which maximizes the packing density of the optical fibers, or optical fiber units, within the cable core, as well as a method of determining the maximum number of optical fibers that can be packed within a cable core within such structure.
What appears to be needed, then, is a fiber optic cable core having a maximized optical fiber packing density, and a method of maximizing the packing density of optical fiber cable cores.
The present invention provides an improved optical fiber cable core design, and a method of specifying the maximum number of optical fiber units that can be provided as a part of this improved cable core, which overcomes some of the deficiencies of the known art. This is accomplished by providing a variable fiber count optical fiber cable core in which the number of optical fibers within the optical fiber units are varied, dependent on the position of the optical fiber units within a stack of optical fiber units comprising the optical fiber cable core. This improved optical fiber cable core design provides a simple, efficient, and cost effective cable core, and method of specifying same, heretofore unavailable in the art. Moreover, the relative simplicity of the cable core of this invention, and the method of determining same, when contrasted to the known optical fiber cable core designs, provides a greater degree of reliability, durability, maintainability, and ease of use while simultaneously increasing the optical fiber packing densities per unit of sheath diameter.
This invention attains this improved packing density, as well as providing simplicity in design and ease of use, by providing a variable fiber count optical fiber cable core for use as a part of an optical fiber cable. The optical fiber cable has an elongate cylindrical core tube formed about a longitudinal axis within which the cable core is received. The cable core is comprised of a stack of plurality of variable fiber count optical fiber units formed symmetrically about the longitudinal axis of the core tube. In one preferred embodiment, the optical fiber units may comprise a plurality of optical fiber ribbons.
The variable fiber count optical fiber cable core of this invention comprises at least a first optical fiber unit having a first predetermined number of optical fibers therein, and at least a second stacked optical fiber unit having a second predetermined number of optical fibers therein, where the first and second predetermined numbers of optical fibers differ within the two optical fiber units so that the number of optical fibers are varied within the optical fiber units which comprise the stack of optical fiber units within the cable core. Moreover, the number of optical fibers within selected ones of the plurality of optical fiber units forming the stack of the cable core may be varied dependent on the position of the selected ones of the optical fiber units within the stack of optical fiber units. Additionally, in fashion heretofore unknown in the art, the variable fiber count optical fiber cable core of this invention provides a stack of optical fiber units which are sized and shaped to approximate a cylindrically shaped stack of optical fiber units within the core tube of the optical fiber cable for maximizing the number of optical fibers that can be safely and efficiently packed within the optical fiber cable core.
Also, it is anticipated that at least some of the optical fibers of at least one of the optical fiber units may be colored differently than others of the optical fibers within the same optical fiber unit for ease of identifying the optical fibers, dependent on, for example, the type of optical fiber being used. For example, the optical fibers may comprise multi mode, single mode or match clad optical fibers, the colors being used to identify which optical fibers are present for ease of use during installation and cable splicing.
In the improved optical fiber cable core of this invention, each of the at least a first and a second optical fiber units, respectively, comprises a first predetermined number of optical fiber sub-units joined together as a single optical fiber unit, each of the optical fiber sub-units being comprised of a second predetermined number of optical fibers. The first predetermined number of optical fiber sub-units, and a second predetermined number of optical fibers within each sub-unit each comprise integers. In one preferred embodiment, this second predetermined number of optical fibers for each such sub-unit will be at least two; and in a second embodiment, there will be at least twelve optical fibers within each such optical fiber sub-unit.
Additionally, at least some of the optical fibers within at least one of the optical fibers sub-units may be colored differently than the remainder of the optical fibers within the same sub-unit, and/or the other sub-units which together comprise the optical fiber unit.
A method of maximizing the packing density of an optical fiber cable core is also provided as a part of this invention. The optical fiber cable core is produced by stacking a plurality of optical fiber units within the core tube of the optical fiber cable, while varying the number of optical fibers within selected ones of the plurality of optical fiber units. This step of varying the number of optical fibers within the optical fiber units occurs in response to the position of selected ones of the optical fiber units within the stack of optical fiber units. The method of this invention also includes the step of establishing a first predetermined number optical fiber sub-units within each optical fiber unit, and multiplying this number of optical fiber sub-units by a second predetermined number of optical fibers for each such sub-unit. Both the first and second predetermined numbers of optical fiber sub-units and optical fibers, respectively, each comprise an integer.
It is, therefore, an object of the present invention to provide an improved optical fiber cable core.
It is another object of the present invention to provide an improved optical fiber cable core in which the packing density of the optical fibers within the cable core is maximized.
It is yet another object of the present invention to provide an improved optical fiber cable core which is simple in design and construction, is rugged and durable in use, and which is easy to use and maintain.
It is to these objects, as well as to the other objects, features and advantages of the present invention, which will become apparent upon reading the specification, when taken in conjunction with the accompanying drawings, to which the invention is directed.